1. Field of the Inventions
The present inventions relate generally to the formation of lesions in tissue.
2. Description of the Related Art
There are many instances where electrodes are inserted into the body. One instance involves the treatment of cardiac conditions such as atrial fibrillation, atrial flutter and ventricular tachycardia, which lead to an unpleasant, irregular heart beat, called arrhythmia. Atrial fibrillation, flutter and ventricular tachycardia occur when anatomical obstacles in the heart disrupt the normally uniform propagation of electrical impulses in the atria. These anatomical obstacles (called “conduction blocks”) can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called “reentry circuits,” disrupt the normally uniform activation of the chambers within the heart.
A variety of minimally invasive electrophysiological procedures, employing catheters and other electrophysiology apparatus that carry one or more electrodes, have been developed to treat conditions within the body by ablating soft tissue (i.e. tissue other than blood and bone). With respect to the heart, minimally invasive electrophysiological procedures have been developed to treat atrial fibrillation, atrial flutter and ventricular tachycardia by forming therapeutic lesions in heart tissue. The formation of lesions by the coagulation of soft tissue (also referred to as “ablation”) during minimally invasive surgical procedures can provide the same therapeutic benefits provided by certain invasive, open-heart surgical procedures.
Tissue charring due to overheating, as well as thrombus and coagulum formation, are sometimes associated with soft tissue coagulation. In order to, among other things, prevent tissue charring and thrombus/coagulum formation, while enabling deeper lesion formation, a variety of electrophysiology systems employ fluid to cool the electrode (or electrodes) and/or the tissue adjacent to the electrodes. In some systems, which are sometimes referred to as “open irrigation systems,” conductive fluid (e.g. saline) exits the electrophysiology device through outlets in the catheter shaft and/or outlets in the electrode. The conductive fluid cools the electrode and adjacent tissue to prevent charring. The conductive fluid also prevents thrombus formation by diluting the blood that comes into contact with the electrode, and also prevents coagulation of blood on the electrode. In some systems, conductive fluid is supplied to the catheter at a constant and relatively high flow rate (e.g. 20-30 ml/min.) during tissue coagulation. In others, this relatively high flow rate may be increased during the coagulation procedure in order to maintain a preset tissue temperature.
The present inventors have determined that conventional open irrigation systems are susceptible to improvement. For example, the present inventors have determined that conventional open irrigation systems can supply too much conductive fluid to the patient, which is sometimes referred to as “fluid overload,” and cause hemodilution or other adverse results.